The present invention relates to a process for the selective and continuous preparation of acetaldehyde from acrolein.
A considerable proportion of acetaldehyde is used for the preparation of acetic esters. For example, ethyl acetate is prepared in a rearrangement reaction with aluminium alcoholates as a catalyst (Claisen-Ti{hacek over (s)}{hacek over (c)}enko reaction). A substantial proportion is also used with formaldehyde for the production of pentaerythritol, an intermediate in the production of alkyl resins and plasticizers and emulsifiers. Furthermore, acetaldehyde is an intermediate in the preparation of butadiene starting from acetylene via acetaldol and its hydration product 1,3-butanediol. The formation of the acetaldol, which can be dehydrated to give crotonaldehyde, takes place by aldol addition of acetaldehyde. The reaction of acetaldehyde with nitrogen compounds to give pyridine and derivatives thereof is becoming increasingly important. Thus, 5-ethyl-2-methylpyridine is prepared in a liquid-phase reaction from acetaldehyde and ammonia. Addition of formaldehyde or acrolein leads to the formation of pyridine and alkylpyridines. Acetaldehyde is further used in the production of peracetic acid, in the oxidation with nitric acid to give glyoxal or glyoxalic acid and in the addition reactions with hydrocyanic acid to give lactonitrile, a precursor of acrylonitrile, and acetic anhydride to give ethylidene diacetate, an intermediate in the vinyl acetate process [Eck2007].
According to the prior art, acrolein can be prepared, inter alia, from the dehydration of glycerol, which occurs as the “waste product” in relatively large amounts in biodiesel production, in near-critical and supercritical water of the addition of acids [Wat2007] or salts [Ott2006]. In the reaction of the resulting acrolein with ammonium salts, acetaldehyde is obtained in high yields.
It is known that acetaldehyde is obtained with a yield of 26% in the dehydration of glycerol in supercritical water at 500° C. and 34.5 MPa and with a residence time of 90 s [Ant1985]. A free radical mechanism by dehydration of the glycerol to 3-hydroxy-propionaldehyde and homolytic cleavage of this intermediate to give acetaldehyde and formaldehyde is assumed for the stated conditions. An alternative mechanism via homolytic cleavage of acetol can be ruled out. The selectivity with respect to acetaldehyde is lower under near-critical water conditions at 360° C. and the same conditions. Addition of sodium hydrogen sulphate as acidic catalyst merely leads to an increase in the yield of acrolein, which presumably results from the acid-catalysed dehydration of 3-hydroxypropionaldehyde.
It is furthermore known that acetaldehyde forms as a by-product with the dehydration of glycerol in near-critical water at between 300 and 350° C. and 34.5 MPa by addition of 0.005 M sulphuric acid [Ant1987]. The reaction of a 0.5 M glycerol solution at 325° C. and with a residence time of 39 s leads to a molar yield of acetaldehyde of only 5%. A retro-aldol reaction starting from acrolein via 3-hydroxypropionaldehyde is assumed as a reaction mechanism, formaldehyde additionally forming and decomposing into hydrogen, carbon monoxide and carbon dioxide under the stated conditions. This assumption was confirmed by the use of an equimolar amount of acetaldehyde and formaldehyde and the cross-aldol reaction to give acrolein. The reaction of a dilute acetaldehyde solution under non-catalytic or alkaline conditions leads mainly to the formation of crotonaldehyde. Furthermore, acetaldehyde can be obtained as the main product by acid-catalysed dehydration from ethylene glycol. The dehydration of a 0.5 M ethylene glycol solution at 385° C. and 34.5 MPa and with a residence time of 29 s leads to a molar acetaldehyde yield of 40%. A disadvantage is the corrosive property of the sulphuric acid, especially in near-critical water.
Further investigations into the dehydration of polyols under supercritical water conditions (385° C., 34 MPa, 20-45 s residence time) are also known [Ram1987]. The acid-catalysed reaction of a 0.5 M ethylene glycol solution leads to a maximum acetaldehyde yield of 41% with 45 s residence time. Hydrogen, carbon monoxide, carbon dioxide and ethylene can be identified as by-products in small amounts. In the dehydration of glycerol in the near-critical range, acetaldehyde is obtained with a maximum yield of 12% at 350° C. and with a residence time of 25 s and addition of catalytic amounts of sulphuric acid. The back-reaction or cross-aldol reaction of acetaldehyde with formaldehyde to give acrolein leads to an acrolein yield of 22%, based on formaldehyde. Crotonaldehyde, which is formed by aldol reaction of acetaldehyde, is detected as a further liquid product. The aldol reaction can be slowed down by addition of acids.
The results of the reaction of glycerol without addition in the near-critical and supercritical water in a temperature range of 250-475° C., at pressures of 25, 35 or 45 MPa and with residence times of 32-165 s and different starting concentration of glycerol are likewise known [Büh2002]. Lower temperatures and higher pressures and longer residence times lead to higher relative selectivities, based on acetaldehyde, the maximum conversion of glycerol being relatively low at 31%. Two competing reaction paths are described for the reaction of glycerol. Ionic reaction steps are preferred at higher pressures and/or lower temperatures, whereas free-radical reactions take place at lower pressures and/or higher temperatures. Acetaldehyde can be formed by both routes and is the main product under all conditions. The reaction mechanisms described for the formation of acetaldehyde differ here from the reaction routes assumed to date. The complete reaction model and the kinetic parameters of the reaction of glycerol can, after optimization, be adapted to the measured data obtained.
It is furthermore known that acetaldehyde forms in the homogeneously catalysed dehydration of ethylene glycol in near-critical and supercritical water [Ott2005]. Thus, acetaldehyde can be obtained with a yield of 10% with addition of catalytic amounts of zinc sulphate to a dilute ethylene glycol solution. By using 20 mM sulphuric acid as a catalyst, the yield can be increased to about 80% at 400° C. and 34 MPa and with a residence time of 15 s. Moreover, zinc sulphate catalyses the subsequent reactions of acrolein from the dehydration of glycerol. For an aqueous 1% (g g−1) acrolein solution, the conversion is 62% at 360° C. and 34 MPa and with a residence time of 120 s. Liquid reaction products cannot be found. Once again, the corrosiveness of the sulphuric acid under the stated conditions is disadvantageous.
In addition, it is known that acetaldehyde is obtained in the dehydration of very dilute aqueous glycerol solutions without addition or addition of sulphuric acid under the near-critical and supercritical conditions in a batch or flow-tube reactor [Wat2007]. The maximum yield of acetaldehyde is about 23% for the continuous dehydration of a 0.05 M glycerol solution at 400° C. and 34.5 MPa and with a residence time of 20 s and addition of 5 mM sulphuric acid. The yields without catalyst are significantly lower. The low starting concentrations of glycerol in combination with the use of sulphuric acid as a catalyst are disadvantageous.